The maximum torque that the dipole experiences when placed in an external electric field is its dipole moment. This is a vector quantity, and the torque is a maximum when the dipole moment is at right angles to the electric field.

A charged rod will attract an uncharged pith ball since it induces a dipole moment in the uncharged pith ball, which now has a dipole moment, is attracted in the inhomogeneous field surrounding the charged rod. How may a dipole moment be induced in an uncharged body? Well, if the uncharged body is metallic, then once placed in an electric field, the free electrons are attracted to one end of the metal, leaving an excess of positive charge at the other end; this induces a dipole moment.

As you may expect from the title of this section, this will be the most difficult and complicated section of this chapter so far. Our aim will be to calculate the field and potential surrounding a simple dipole.

While a single charge is a scalar quantity, and a dipole moment is a vector quantity, the quadrupole moment is a second order symmetric tensor. It has no net charge and no net dipole moment. Unlike a dipole, it will experience neither a net force nor a net torque in any uniform field. It may or may not experience a net force in an external nonuniform field

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